This invention relates to a magnetic disk to be loaded in a magnetic disk apparatus, such as a hard disk drive.
Following the rapid development of the IT (information technology) industry in recent years, dramatic technical innovation is required in the information recording technology, in particular, in the magnetic recording technology. For example, in a magnetic disk to be loaded in a hard disk drive (hereinafter will be referred to as a HDD), a technique capable of achieving an information recording density on the order of 40 Gbit/inch2 to 100 Gbit/inch2 or more is required in order to meet the demand for a higher storage capacity.
Following the above-mentioned demand for a higher storage capacity, the magnetic disk is required to be excellent particularly in magnetic characteristic in a flying/tracking direction of a magnetic head of the HDD. For this purpose, it is proposed in Japanese Patent Application Publication (JP-A) No. S62-273619 to deposit a magnetic layer on a metal substrate (for example, an aluminum alloy substrate) as a substrate for a magnetic disk after a texture for inducing magnetic anisotropy in the magnetic layer is formed on the surface of the substrate. With this structure, it is possible to improve the magnetic characteristic in the flying/tracking direction of the magnetic head in comparison with the magnetic characteristic in a radial direction.
On the other hand, in order to meet the recent demand for a mobile and small-sized HDD, attention is recently drawn to a glass substrate for use as the substrate for a magnetic disk. The glass substrate is high in rigidity, excellent in shock resistance, and high in surface flatness and smoothness. Since the glass substrate is excellent in shock resistance by replacing the aluminum alloy by the glass as the material of the magnetic disk, it is unnecessary in a magnetic disk production process to enhance the rigidity by coating the substrate with a metal film such as a NiP film as required in the aluminum alloy substrate. As a result, the magnetic disk production process is shortened and the magnetic disk is provided at a low cost. In addition, the HDD is easily reduced in size. For example, Japanese Patent Application Publication (JP-A) No. 2002-32909 proposes a magnetic recording medium comprising a glass substrate provided with a circular texture and a magnetic layer formed on the substrate by sputtering.
In case where the glass substrate is used as the substrate for a magnetic disk, it is desired, like in the aluminum alloy substrate, that the magnetic characteristic in a circumferential direction as the flying/tracking direction of the magnetic head is more excellent than that in the radial direction.
In order to evaluate the magnetic characteristics of the magnetic disk in the radial direction and in the circumferential direction, use is made of an oriented ratio of magnetic anisotropy (hereinafter referred to as MrtOR) calculated from a residual magnetization-thickness product.
MrtOR represents the oriented ratio (OR) of magnetic anisotropy calculated from the residual magnetization-thickness product (Mrt). At any given point on a principal surface of a magnetic recording medium such as a magnetic disk, Mrt in the circumferential direction is represented by Mrt(c) while Mrt in the radial direction is represented by Mrt(r). MrtOR is defined as Mrt(c)/Mrt(r) as a ratio of Mrt(c) with respect to Mrt(r). Mrt is a product of Mr (residual magnetization) and t (thickness of the magnetic layer of the medium).
If MrtOR is substantially equal to 1 in a magnetic recording medium such as a magnetic disk, the magnetic characteristic in the circumferential direction is substantially same as that in the radial direction. In this event, the magnetic recording medium is isotropic. As MrtOR becomes greater beyond 1, the magnetic anisotropy as the magnetic characteristic in the circumferential direction is improved or increased.
In order to achieve a recording density of 40 Gbit/inch2 or more in the magnetic disk, it is required that MrtOR is not smaller than 1.2. In order to achieve a recording density of 50 Gbit/inch2 or more, it is desired that MrtOR is not smaller than 1.3. In order to achieve a recording density of 60 Gbit/inch2 or more, it is desired that MrtOR is not smaller than 1.35.
MrtOR in a desired range can be obtained in case where an aluminum alloy substrate or a substrate coated with a metal film such as NiP is used as a substrate for a magnetic disk, a texture for inducing magnetic anisotropy is formed on its metallic surface, and a magnetic layer is formed on the substrate.
On the other hand, in case where a glass substrate is used as a substrate for a magnetic disk, a texture for inducing magnetic anisotropy is formed directly on its glass surface, and a magnetic layer is formed on the substrate, MrtOR is no more than 1.0 to 1.1. Thus, the glass substrate is not appropriate for a higher recording density. As a consequence, the glass substrate can not be used as a substrate for a magnetic disk in order to achieve the objects of a reduction of the HDD in size, an increase in storage capacity, and a decrease in cost. Thus, the advantages of the glass substrate, such as the excellent shock resistance, the high surface flatness and smoothness, and the cost saving effect can not be utilized. This constitutes a factor of inhibiting achievement of the above-mentioned objects.